Method and apparatus for electrostatic spraying

ABSTRACT

One or more auxiliary electric fields are established for an electrostatic powder sprayer to further control the movement of the powder particles from the area of the usual primary electric field to the surface to be coated, so that a more uniform powder profile is laid down on the surface to be coated and the amount of powder fly around is reduced.

United States Patent Reilly [54] A METHOD AND APPARATUS FORELECTROSTATIC SPRAYING [72] Inventor: Dennis A. Reilly, Berkeley, Calif.

[73] Assignee: Oxy-Dry Chicago, Ill.

[22] Filed: Oct. 5, 1970 [21] Appl. No.: 78,025

Sprayer Corporation,

[52] US. Cl. ..239/3, 117/17, 117/93.4, 118/623, 239/15, 317/3 [51] 1.1.CI. ..B05b 5/00 [58] Field of Search ..239/3, 15; 117/93.4, 17; 118/621,623, 629, 630, 638; 317/3 [56] References Cited UNITED STATES PATENTS2,608,176 8/1952 Jenkins et al. ..117/93.4 X 2,754,225 7/1956 Gfeller..239/3 X 2,082,182 6/1937 Schacht ..1l7/93.4 X

[151 3,680,779 [451 Aug. 1,1972

Primary Examiner-M. Henson Wood, Jr.

Assistant Examiner-Edwin D. Grant Attomey-Wolfe, Hubbard, Leydig, Voit &Osann [57] ABSTRACT One or more auxiliary electric fields areestablished for an electrostatic powder sprayer to further control themovement of the powder particles from the area of the usual primaryelectric field to the surface to be coated, so that a more uniformpowder profile is laid down on the surface to be coated and the amountof powder fly around is reduced.

19 Claims, 9 Drawing Figures momma a M a. f; .4 h

METHOD AND APPARATUS FOR ELECTROSTATIC SPRAYING BACKGROUND OF THEINVENTION This invention relates to electrostatic sprayers, and moreparticularly to an improved method and apparatus for laying down auniform coating on a moving surface. The advantages of the instantinvention are realized to a high degree in the application thereof to lanother or from one part of the web to another when the sheets arestacked or the web is folded. Various forms of powders, commonlyreferred to as anti-offsetting agents, and powder spraying mechanismshave been used. However, for modern high speed printing operations,electrostatic powder sprayers have been most widely employed inconnection with an anti-offsetting agent, such as highly particulatestarch or the like.

The known electrostatic sprayers are characterized by including ametering roller for dispensing the powdered anti-offsetting agent at asubstantially uniform rate and one or more dielectric sheathedelectrodes or glass housed gas discharge tubes (hereinafter sometimescollectively referred to for convenience as insulated conductors) whichare spaced from the roller up to 1 cm. or so. The metering rollerdispenses the powder with a geometry generally corresponding to thecross-sectional geometry of the desired powder lay down profile (e.g., aline geometry). The insulated conductors are axially aligned with themetering roller and an alternating current field is established betweenthem and the metering roller. The field is of sufficient strength, i.e.,has a sufficient rms voltage gradient, to ensure ionization of each ofthe air spaces between the metering roller and the insulated conductors.Consequently, the individual particles of powder presented by themetering roller assume electrical charges that correspond in polarity tothe existing polarity of the ac. field. Further, it has been found thatdue to the convergence of the ac. field toward the insulated conductoror conductors there is a dielectric force tending to strip powderparticles from the metering roller and accelerate them towards theinsulated conductor or conductors. During the existing or first halfcycle of the field, the particles are, therefore attracted toward theinsulated conductors. However, during the next or succeeding half cycleof the field, when the field polarity reverses, the charged particlesare repelled from the insulated conductors, and most of them areaccelerated out of the influence of the field to fall under the force ofgravity. Ideally, the free falling particles of powder settle to form acoating with a uniform profile on a freshly printed sheet or web passingbeneath the sprayer at the time.

Unfortunately, a great deal of difficulty has been experienced with theair turbulence and drafts that are inevitably present during theoperation of electrostatic sprayers, particularly when such sprayers areused in conjunction with high speed printing operations. Such aircurrents have a very substantial effect on the free falling powder andcause a significant departure from the ideal uniform powder laydown,since the powder particles are very lightweight and extremely small, sayon the order of to 80 microns. Elaborate precautions, such as theinclusion of air curtains and the like, have been taken to eliminate theso-called powder flyaround caused by the air movement, but these arequite costly and have not been altogether successful, since many of thesources of disturbing air currents are 0 in the immediate vicinity ofthe sprayer itself.

SUMMARY OF THE INVENTION A general object of the present invention is toprovide methods and apparatuses for electrostatic spraying which provideincreased control over the laydown of a sprayed coating.

Another object of the present invention is to provide a method andapparatus for electrostatic powder spraying which minimizes powderfly-around. A related object is to provide an electrostatic powderspraying method and apparatus which minimizes the amount of powderrequired to lay down a powder coating with a desired profile. A morespecific related object is to provide a method and apparatus of theforegoing type wherein the powder laydown is substantially undisturbedby normal ambient air currents.

Still another object of this invention is to provide a method andapparatus for electrostatic powder spraying wherein there is increasedcontrol over the powder motion. A related and more specific object is toprovide a method and apparatus of the foregoing type which is suitablefor electrostatic powder spraying of articles traveling above or belowthe sprayer, or vertically, or with any other orientation relativethereto that may be desired. A further object of the instant inventionis to provide an improved method and apparatus for electrostatic powderspraying which may be readily incorporated into existing equipment, aswell as new equipment, at a relatively low cost.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of thepresent invention will become apparent as the following detaileddescription is read in conjunction with the attached drawings, in which:

FIG. 1 is a simplified perspective view of a typical electrostaticpowder sprayer with which the present invention may be advantageouslyemployed;

FIG. 2 is a sectional view of the sprayer taken along the line 2-2 inFIG. 1 and incorporates an electrical schematic diagram to illustrateone embodiment of the invention;

FIG. 3 is partly in sectional and partly in schematic form to showanother embodiment of the invention;

FIG. 4 illustrates the advantageous combination of the embodiments ofthe invention depicted in FIGS. 2 and 3;

FIG. 5 generally corresponds to FIG. 2, but shows that the orientationof the article to be coated relative to the electrostatic powder sprayerof the instant invention is immaterial;

FIG. 6 is partly in sectional and partly in schematic form to illustratean embodiment of the invention wherein the article to be coated is usedas an electrode;

FIG. 7 shows the sprayer in sectional form and includes a simplifiedelectrical diagram of one circuit for operating the sprayer with apotential difference between its metering roller and ground;

FIG. 8 generally corresponds to FIG. 6 but the electrical diagram ismodified to illustrate another and more economical circuit for operatingthe sprayer with a potential difference between its metering roller andground;

FIG. 9 is a sectional view of the sprayer, together with an electricaldiagram of a circuit for deriving the voltages required by any one ofthe various embodiments of the instant invention from a single source.

I DETAILEDDESCRIPTION While the invention is described hereinafter inconnection with certain illustrated embodiments, it is to be understoodthat the intent is not to limit it to those embodirnents. To thecontrary, the intent is to cover all modifications, equivalents andalternatives as are within the spirit and scope of the invention asdefined by the appended claims.

Turning to the drawings, and particularly to FIGS. 1 and 2, theelectrostatic powder sprayer shown includes a powder box or reservoir 11which has an open lower side within which a metering roller 12 isdisposed. Mounting brackets 13 and 14 are secured to the powder box 11at its opposite ends. The metering roller 12 is journalled on themounting brackets 13 and 14 and driven by a variable speed motor 15through a speed reduction box 16 and a shaft coupling 17.

In keeping with accepted practices, the circumference of the meteringroller 12 is spaced from the inner surfaces of the powder box 11 andprepared, by etching, grinding or the like, to comprise numerous closelyspaced pockets or grooves in which the powder 21 to be dispensedcollects. Further, for ensuring that the powder 21 is dispensed from thepowder box 11 at a substantially uniform rate and with a distribution orgeometry corresponding to the cross-sectional line geometry desiredforpowder laydown profile, there are a pair of spring-like wiper blades 22and 23 secured (by means not shown) to the inner surfaces of the powderbox 11. The wiper blades 22 and 23 tangentially engage thecircumferenceof the metering roller 12 along lines that are slightlybelow and respectively fore and aft of the axis of revolution of themetering roller 12. Of course, the rate at which the powder 21 isdispensed from the powder box 11 depends on the speed at which themetering roller 12 is driven, together with the pressure exerted againstthe metering roller 12 by the wiper blades 22 and 23.

In the particular embodiment shown in FIGS. 1 and 2, a primary electricfield is established by an ac. voltage source 25 which is connected atone side to the roller 12 and at its other side to a pair of dielectricsheathed electrodes or glass housed gas discharge tubes (i.e., insulatedconductors) 26 and 27. However, it is to be understood that while it ispreferable and advantageous that one or the other of the metering roller12 or conductors 26 and 27 be insulated to prevent arc dischargingtherebetween, it is not absolutely essential to the broader aspects ofthe invention. Typically, the metering roller 12 is grounded asindicated at 28. The conductors 26 and 27 are respectively located foreand aft of the axis of rotation of the metering roller 12, and each ofthem is supported on insulated straps 29 or the like by the powder box11. Suitably the distance between the circumference of the meteringroller 12 and each of the insulated conductors 26 and 27 is on the orderof about 1 cm.

As is generally the case in electrostatic powder spraying, the ac..voltage source 25 has an rms output voltage on the order ofapproximately 10 kv., so that the air spaces between the metering roller12 and the insulated conductors 26 and 27 are ionized (i.e., areas ofcorona discharge activity). Further, the ac. field between the meteringroller 12 and conductors 26 and 27 converges or intensifies in the areaof the conductors 26 and 27 so that there is a dielectric force actingon the particles which tends to accelerate them towards the conductors26 and 27 independently of the polarity of the field or the presence ofa charge on the particles. Thus, the electrical forces are alonesufficient to ensure a flow of particles from the metering roller 12,and additional accelerators such as air jets or the like, which mightdisturb the uniformity of the powder distribution, are not required.Hence, as will be seen, during one half cycle of the ac. voltagesupplied by the source 25, the particles of powder 21 presented to theionized areas by the metering roller 12 and under the influence of thedielectric force assume changes that correspond in polarity to thepolarity of the existing half cycle of the field, and the particles are,therefore, attracted toward the insulated conductors 26 and 27. Duringthe next or opposite polarity half cycle, the charged powder particlesare repelled by the insulated conductors 26 and 27, with the result thatmost of them are accelerated out of the influence of the primaryelectric field. As previously noted, in conventional electrostaticpowder spraying, the powder particles that are accelerated out of theinfluence of the electric field established by the ac. voltage source 25fall freely under the force of gravity toward the surface 31 to becoated, such as the freshly inked surface of a sheet or web, as it moveunder the sprayer, such as on a conveyor or the like generally indicatedat 32. However, due to the inevitably air currents, substantialquantities of powder are blown off course to create the aforementionedpowder fly-around problem.

Thus, in accordance with the present invention, one or more auxiliaryelectric fields are established to further control the movement of thepowder particles from the area of the metering roller 12 to the surface31 to be coated. As will be appreciated, particularly afterconsideration of the various embodiments here disclosed, the auxiliaryfield or fields may be established in a variety of different ways toafford various degrees of control. However, in each instance, thefurther control provided aids in achieving a more uniform powder laydownprofile on the surface 31 to be coated, while at the same timesubstantially reducing the quantity of powder involved in whateverpowder fly-around may still exist.

For example, as shown in FIG. 2, to provide a secondary electric fieldthere may be a voltage source 34 that is connected between the meteringroller 12 and an electrode 35 positioned beneath the surface 31 to becoated. The electrode 35 may be in a variety of differentconfigurations, such as a bar, sheet, or mesh. But

whatever its form may be, its longitudinal axis preferably lies parallelto the axis of rotation of the metering roller 12 to ensure that theauxiliary field is substantially uniformly distributed. The voltagesource 34 may be either a d.c. source or an a.c. source. In eitherevent, however, the effective voltage level of the source 34, i.e., d.c.voltage level in the case of a d.c. source and rms voltage level in thecase of an a.c. source, is low enough to ensure that the, primary fieldpredominates, but high enough to ensure that a substantial majority ofthe powder particles accelerated out of the primary field are capturedby and come under the influence of the secondary field. For a typicalelectrostatic powder spraying set-up, wherein there is a distance of afew inches between the metering roller 12 and the surface 31 to becoated, a suitable effective voltage level for the voltage source 34 ison the order of, say, 18-20 kv.

The powder particles that come under the influence of the secondaryfield are constrained thereby and accelerated toward either the surface31 to be coated or the metering roller 12 in dependence on the polarityof the particle charge relative to the existing polarity of thesecondary field. Preferably the secondary field has a frequency that islow relative to that of the primary field to afford sufficient time fora quantity of powder particles to travel from the area of the insulatedconductors 26 and 27 to the surface 31 to be coated without anintervening reversal of the polarity of the secondary field. Due to theconstraining influence of the secondary field, ambient air currents havelittle, if any, effect on the distribution of the powder particles.Thus, the powder particles that are accelerated toward the surface 31form a statistically uniform profile coating on it, whereas theparticles that are accelerated toward the metering roller 12 arerecycled. There is, therefore, very little powder involved in any powderfly-around.

The flow of powder 21 from the metering roller 12 is literally pulsed onand off at a frequency that is determined by the rate at which therelative polarities of the primary and secondary fields change. In theevent that the voltage source 34 is a d.c. source, the rate at which therelative polarities of the primary and secondary fields change isdetermined by the frequency of the a.c. source 25. On the other hand, inthe event that the voltage source 34 is an a.c. source, the same rate isdetermined by the frequencies of the sources 25 and 34. In either case,the determinative frequency or frequencies can be readily selected sothat the relative polarities of the primary and secondary fields changeat a rate selected to turn the powder flow on and off in timedsynchronism with the rate at which successive articles 31 to be coatedare moved past the sprayer. Thus, the instant invention affords a way toobtain an extremely high degree of effective utilization of theavailable powder supply, even when the articles 31 to be coated areseparated on the conveyor 32.

In the embodiment of FIG. 2 the secondary field extends from themetering roller 12 to below the surface 31 to be coated. However, thedisturbing effects of ambient air currents can also be substantiallyreduced by a secondary field which extends only part of the way betweenthe metering roller 12 and the surface 31. Accordingly, in keeping withthe broader aspects of the present invention, in the embodiment of FIG.3 a voltage source 36 is connected between the metering roller 12 andwhat, in this instance, are shown as being a set of parallel,equidistantly spaced, bar-type electrodes 37a-37. The primary advantageof this embodiment is that there is an increased uniformity of thesecondary electric field due to the decreased distance between themetering roller 12 and the electrodes 37a-37d which, in turn, increasesthe uniformity of the distribution of the powder particles. However,there is not the same degree of constraint for the powder particles asin the embodiment of FIG. 2, since they leave the influence of thesecondary field before being deposited on the sur face 31.

Again, the source 36 may be either a d.c. or an a.c. source. However, ifit is a d.c. source, the powder particles that are traveling toward thesurface 31 decelerate once they pass the plane of the electrodes37a-37d, and are, therefore, increasingly subject to the disturbingeffects of ambient air currents. This drawback can be alleviated to anextent, generally at the expense of sacrificing the capability ofsynchronizing the powder flow with the movement of successive articlespast the sprayer, by using an a.c. source with characteristics asdescribed below for the voltage source 36. More particularly, with ana.c. source 36 to create the secondary field, its frequency can be setrelative to the frequency of the voltage source 25 so that the rate atwhich the polarity of the secondary field is reversed is synchronizedwith the rate at which the polarity of the primary field is reversed,and its phase angle may be set relative to the phase angle of the a.c.source 25 so that the secondary field polarity reversal lags the primaryfield polarity reversal by a period that is substantially equal to thetransit time for the powder particles from the metering roller 12 to theplane of the electrodes 37a-37. As will be appreciated, the result ofthe foregoing is that the powder particles 21 are repelled as they passthe electrodes 37a-37d and, therefore, accelerated toward the surface 31to be coated, rather than being retarded. Of course, the additionalacceleration makes the powder particles less susceptible to thedisturbing efiects of the ambient air currents.

The advantageous features of the total constraint afforded by theembodiment of FIG. 2 and the more uniform field distribution of FIG. 3may be readily combined, as shown in FIG. 4. Thus, the voltage source 34and the electrode 35 may be used to create a secondary field extendingfrom the metering roller 12 to below the surface 31 to be coated, andthe voltage source 36 may be used to create a tertiary field in theproximity of the metering roller 12. Both of the voltage sources 34 and36 may be either a.c. or d.c. sources, or one of them may be an a.c.source and the other a d.c. source. If both sources 34 and 36 are d.c.sources, they are selected so that the electrodes 37a-37d are held at apotential intermediate the potentials at which the metering roller 12and the electrode 35 are held. On the other hand, if either or both ofthe sources 34 and 36 are a.c. sources, they are selected so that thepotential of the electrodes 37a-37d is at least periodically, if notalways, intermediate the potentials of the metering roller 12 and theelectrode 35. Of course, when the potential of the electrodes 37a-37d isintermediate the potentials of the metering roller 12 and the electrodeare identical or complimentary. Furthermore, the

powder particles that are traveling toward the surface 31 to be coateddo not decelerate as they pass the plane of the electrodes 37a-37d.Rather, they then come under the constraining influence of the secondaryfield and are further accelerated thereby toward the surface 31 to becoated.

As will be appreciated in the embodiment of FIG. 4 the flow of powderparticles 21 from the metering roller 12 may be turned on and off insynchronism with the movement of successive articles to be coated pastthe powder sprayer. Such synchronism can be affected most efficiently byusing an a.c. source 34 to establish the secondary field and byselecting its frequency so that the relative polarities of the primaryand secondary fields change in synchronism with and at the rate at whichsuccessive articles are moved past the sprayer. Substantially the sameresult may, of course, also be obtained by using an a.c. source 36 tocreate the tertiary field and by selecting its frequency so that therelative polarities of the primary and tertiary fields change at therate with which successive articles are moved past the sprayer, but witha leading phase angle sufficient to compensate for the transit time ofthe powder particles from the plane of the electrodes 37a-37d to theplane in which the articles are moved past the sprayer. The lastmentioned method for synchronously pulsing the powder flow on and off isless efficient than the first mentioned simply because some of thepowder particles that are repelled toward the metering roller 12 by thechange of polarity of the tertiary field relative to the primary fieldmay nevertheless be captured by the secondary field and acceleratedthereby away from the metering roller.

Another of the more unusual features of the present invention is thatthe orientation of the surface 31 to be coated relative to the powdersprayer is not critical. As previously noted, prior an electrostaticpowder sprayers have relied on gravity to carry the powder particles andhave, therefore, been limited to coating articles that are passed belowthem. However, with a secondary field to constrain the particles ofpowder and accelerate them toward the surface 31 to be coated, theorientation of the surface 31 relative to the sprayer is not critical,and may be such that the gravitational forces actually tend to deflectthe powder particles away therefrom (i.e., the secondary or auxiliaryfield forces may be angularly displaced from the gravitational forces).For example, as shown in FIG. 5, the surface 31 may be located above thesprayer, since the secondary field between the metering roller 12 andthe electrode 35 permits the powder particles to be accelerated towardthe surface 31 against the gravitational force. For the particularembodiment shown, the primary field is preferably created between themetering roller 12 and a single insulated conductor 38 that is disposedintermediate the metering roller 12 and the surface 31. Otherwise, theembodiment of FIG. generally corresponds to and has the same features asthe embodiment of FIG. 2.

If the article to be coated is sufficiently effective as a dielectric,say a plastic material or the like, the article 31 may itself be used asan electrode for the secondary field. To this end, as shown in FIG. 6, acharge is deposited on the article 31 at the input side or upstream ofthe powder sprayer, such as by exposing it to a corona discharge from anelectrode 41 that is fed by a voltage source 42. As the charged article31 is moved beneath the powder sprayer, the voltage gradient between themetering roller 12 and the article 31 to be coated creates a secondaryfield in much the same manner and with the same characteristics aspreviously described in connection with the embodiment of FIG. 2. Aconventional static eliminator 43 may be disposed at the output side ordownstream of the powder sprayer for discharging the coated article 31,so that the article 31 may be more readily stacked or folded.

Generally, the components of the conveyor 32, such as the gripper bars44 and drive chain 45 (FIG. 2), are conductive and, therefore, assert adistorting influence on the secondary field. However, in keeping withone of the more detailed aspects of the present invention, thedistortion of the secondary field may be minimized by referencing themetering roller 12 and the electrode 35 to potentials that are ofopposed polarities relative to ground potential. To accomplish this-end,while still referencing the a.c. potential used to create the primaryfield to the potential of the metering roller 12, the a.c. voltagesource 25 may be connected as in FIG. 7 across the primary winding 46 ofa d.c. isolating transformer 47 which, in turn, has its secondarywinding 48 connected between the metering roller 12 and one or moreinsulated conductors 49. Alternatively and more economically, the sameeffect may be achieved as in FIG. 8 by connecting the a.c. voltagesource 25 across the metering roller 12 and the insulated conductor 48through respective d.c. isolating capacitors 51 and 52. In that eventthe capacitors 51 and 52 preferably have values on the order of ten ormore times the capacitance between the metering roller 12 and insulatedconductor 49.

In passing it is noted that the single insulated conductor 49 shown inthe embodiments of FIGS. 7 and 8 is located rearward and slightly belowthe axis of revolution of the metering roller 12. Likewise, in FIG. 9the single insulated conductor is located below but forward of the axisof revolution of the metering roller 12. These showings are made simplyto graphically indicate known possible alternatives for the basicelectrostatic powder sprayer.

Finally, it will be seen that the various voltages required forelectrostatic powder spraying in accordance with the present inventionmay be derived from a single a.c. voltage source 53. For example, FIG. 9illustrates a simplified single source counterpart for the embodiment ofFIG. 3 wherein the secondary and tertiary fields are d.c. fields. Moreparticularly, in the embodiment of FIG. 9, the a.c. source 53 isconnected across the primary winding 54 of a transformer 55. Themetering roller 12 and the insulated conductor 49 are connected toopposite ends or across the secondary winding 56 of the transformer 55by respective d.c. isolating capacitors 57 and 58. To derive the d.c.potentials for the secondary and tertiary fields, there is a full waverectifier, here shown as being a voltage doubler 59, connected acrossthe secondary winding 56 of the transformer 54. Further a voltagedividing potentiometer 60 with a pair of sliders 61 and 62 is connectedacross the full wave rectifier 59. The full d.c.

voltage developed by the full wave rectifier 59 is applied between themetering roller 12 and the electrode 35 to create the secondary field,but only a part of it, as determined by the setting of the voltagedividing slider 61, is applied across the metering roller 12 and theelectrodes 37a-37d to create the tertiary field. Thus, the electrodes37a-37d are held at a potential intermediate the potentials of themetering roller 12 and the electrode 35, so that the powder particlesleaving the tertiary field are captured by the secondary field andaccelerated thereby toward the surface 31 to be coated. The dc.potentials relative to ground at which the metering roller 12,electrode. 35, and electrodes 37a-37 and held are established by thesetting of the grounded slider 62.

CONCLUSION In view of the foregoing, it will now be understood that thepresent invention provides a method and apparatus for electrostaticpowder spraying which ensures increased uniformity of the powder profiledeposited on the surface to be coated, while at the same time minimizingthe amount of powder flyaround. It will also be appreciated that thepresent invention may be used to advantage for spraying articles thathave any desired orientation relative to the sprayer. Finally, it willbe seen that the present invention is entirely compatible with existingelectrostatic powder sprayers and can, therefore, be used with them, aswell as with new equipment.

I claim as my invention:

1. In an electrostatic sprayer for depositing a material on an articlespaced from and moving relative to said sprayer along a predeterminedpath, said sprayer including a metering means for supplying particles ofsaid material, a conductor means spaced at a predetermined distance fromsaid metering means intermediate said metering means and said path, andfirst circuit means connected to said metering means and said conductormeans for establishing a primary a.c. electric field therebetweenwhereby said particles are accelerated away from said metering means;the combination of at least one electrode means spaced at a distancefrom said metering means greater than said predetermined distance, andsecond circuit means connected between said metering means and saidelectrode means for establishing an auxiliary electric fieldtherebetween, said electrode means being positioned relative to saidmetering means and said path such that said auxiliary field furthercontrols movement of said particles between said metering means and saidarticle.

2. The electrostatic sprayer of claim 1 wherein successive articles aremoved relative to said sprayer along said path at a predetermined rate,said auxiliary field is a dc. field, and said primary a.c. field has afrequency selected to synchronize the movement of said particles fromsaid metering means to said path with the rate at which said articlesare moved relative to said sprayer.

3. The electrostatic sprayer of claim 1 wherein successive articles aremoved relative to said sprayer along said path at a predetermined rate,said auxiliary electric field is an ac. field, and said primary andauxiliary a.c. fields have relative frequencies and phases selected tosynchronize the movement of said particles from said metering means tosaid path with the rate at which said articles are moved relative tosaid sprayer.

4. The electrostatic sprayer of claim 1 wherein said path is disposedbetween said metering means and said electrode means, whereby theauxiliary field constrains said particles after they are accelerated outof said primary field and until they are deposited on said article.

5. The electrostatic sprayer of claim 4 wherein successive articles aremoved relative to said sprayer along said path at a predetermined rate,the auxiliary field is an ac. field, and the primary and auxiliaryfields have relative frequencies and phases that are selected to causesaid particles to move toward and away from said metering means in timedsynchronism with the movement of said articles relative to said sprayer.

6. The electrostatic sprayer of claim 1 wherein said path is orientedrelative to said sprayer means such that any gravitational force actingon said particles tends to deflect such particles away from said path.

7. The electrostatic sprayer of claim 1 wherein said electrode means ispositioned between said metering means and said path, whereby theauxiliary electric field constrains said particles after they areaccelerated out of said primary field for only a part of the distancebetween said metering means and said path.

8. The electrostatic sprayer of claim 7 wherein successive articles aremoved relative to said sprayer along said path at a predetermined rate,the auxiliary electric field is an ac. field, and said primary andauxiliary a.c. fields have relative frequencies and phases such thatsaid particles are moved toward and away from said path in timedsynchronism with the movement of said articles relative to said sprayer.

9. The electrostatic sprayer of claim 7 wherein the auxiliary electricfield created is an ac. field, and said primary and auxiliary a.c.fields have relative frequencies and phases such that said particles areaccelerated toward said path by said auxiliary field after passingthrough the distance between said metering means and said firstelectrode means.

10. The electrostatic sprayer of claim 4 wherein said metering means andsaid electrode means are referenced to respective potentials which areof opposite polarities relative to a ground potential.

11. The electrostatic sprayer of claim 10 wherein said electrode meansis a first electrode means, and further including a second electrodemeans which is referenced to a potential which is intermediate thepotentials to which said metering means and said first electrode meansare referenced.

12. The electrostatic sprayer of claim 4 wherein said electrode means isa first electrode means, and further including a second electrode meansdisposed between said metering means and said path, whereby a tertiaryelectric field of increased uniformity relative to said auxiliary fieldis established to provide an increased uniformity of distribution ofsaid particles relative to the article to be coated.

13. The electrostatic sprayer of claim 12 further including a single ac.voltage source which is connected by said first and second circuit meansto said metering means and said first and second electrode means forestablishing said primary, auxiliary, and tertiary electric field.

14. The electrostatic sprayer of claim 1 wherein said article is adielectric, and further including a means positioned at an input side ofsaid sprayer for depositing a predetermined charge on said articlewhereby said auxiliary electric field is established between saidmetering means and said article, and a means positioned at an outputside of said sprayer for discharging said article.

15. The electrostatic sprayer of claim 1 wherein one of said meteringmeans and said conductor means is insulated.

16. A method for electrostatically spraying a powder coating on anarticle comprising the steps of:

continuously supplying particles of powder, exposing said particles in apredetermined area at a distance from said article to an ac. primaryelectric field so that successive groups of particles are oppositelycharged relative to a predetermined reference potential for said primaryfield, and

creating an auxiliary electric field to control movement of saidparticles for at least a portion of said distance.

17. The method of claim 15 wherein said auxiliary electric field extendsfor at least the entirety of said distance, whereby it controls movementof said particles from the area of said primary field to said article.

18. The method of claim 16 wherein said auxiliary electric field assertsa force on said particles in a direction that is angularly, displacedfrom the direction of any gravitational forces acting thereon.

19. The method of claim 15 further including the step of movingsuccessive articles to be coated past a predetermined spraying stationat a predetermined rate; and wherein said auxiliary electric field is anac. field, and said primary and auxiliary fields have relativefrequencies and phases that are selected to cause said powder particlesto move towardand away from said station in timed synchronism with themovement of successive articles thereby.

1. In an electrostatic sprayer for depositing a material on an articlespaced from and moving relative to said sprayer along a predeterminedpath, said sprayer including a metering means for supplying particles ofsaid material, a conductor means spaced at a predetermined distance fromsaid metering means intermediate said metering means and said path, andfirst circuit means connected to said metering means and said conductormeans for establishing a primary a.c. electric field therebetweenwhereby said particles are accelerated away from said metering means;the combination of at least one electrode means spaced at a distancefrom said metering means greater than said predetermined distance, andsecond circuit means connected between said metering means and saidelectrode means for establishing an auxiliary electric fieldtherebetween, said electrode means being positioned relative to saidmetering means and said path such that said auxiliary field furthercontrols movement of said particles between said metering means and saidarticle.
 2. The electrostatic sprayer of claim 1 wherein successivearticles are moved relative to said sprayer along said path at apredetermined rate, said auxiliary field is a d.c. field, and saidprimary a.c. field has a frequency selected to synchronize the movementof said particles from said metering means to said path with the rate atwhich said articles are moved relative to said sprayer.
 3. Theelectrostatic sprayer of claim 1 wherein successive articles are movedrelative to said sprayer along said path at a predetermined rate, saidauxiliary electric field is an a.c. field, and said primary andauxiliary a.c. fields have relative frequencies and phases selected tosynchronize the movement of said particles from said metering means tosaid path with the rate at which said articles are moved relative tosaid sprayer.
 4. The electrostatic sprayer of claim 1 wherein said pathis disposed between said metering means and said electrode means,whereby the auxiliary field constrains said particles after they areaccelerated out of said primary field and until they are deposited onsaid article.
 5. The electrostatic sprayer of claim 4 wherein successivearticles aRe moved relative to said sprayer along said path at apredetermined rate, the auxiliary field is an a.c. field, and theprimary and auxiliary fields have relative frequencies and phases thatare selected to cause said particles to move toward and away from saidmetering means in timed synchronism with the movement of said articlesrelative to said sprayer.
 6. The electrostatic sprayer of claim 1wherein said path is oriented relative to said sprayer means such thatany gravitational force acting on said particles tends to deflect suchparticles away from said path.
 7. The electrostatic sprayer of claim 1wherein said electrode means is positioned between said metering meansand said path, whereby the auxiliary electric field constrains saidparticles after they are accelerated out of said primary field for onlya part of the distance between said metering means and said path.
 8. Theelectrostatic sprayer of claim 7 wherein successive articles are movedrelative to said sprayer along said path at a predetermined rate, theauxiliary electric field is an a.c. field, and said primary andauxiliary a.c. fields have relative frequencies and phases such thatsaid particles are moved toward and away from said path in timedsynchronism with the movement of said articles relative to said sprayer.9. The electrostatic sprayer of claim 7 wherein the auxiliary electricfield created is an a.c. field, and said primary and auxiliary a.c.fields have relative frequencies and phases such that said particles areaccelerated toward said path by said auxiliary field after passingthrough the distance between said metering means and said firstelectrode means.
 10. The electrostatic sprayer of claim 4 wherein saidmetering means and said electrode means are referenced to respectivepotentials which are of opposite polarities relative to a groundpotential.
 11. The electrostatic sprayer of claim 10 wherein saidelectrode means is a first electrode means, and further including asecond electrode means which is referenced to a potential which isintermediate the potentials to which said metering means and said firstelectrode means are referenced.
 12. The electrostatic sprayer of claim 4wherein said electrode means is a first electrode means, and furtherincluding a second electrode means disposed between said metering meansand said path, whereby a tertiary electric field of increased uniformityrelative to said auxiliary field is established to provide an increaseduniformity of distribution of said particles relative to the article tobe coated.
 13. The electrostatic sprayer of claim 12 further including asingle a.c. voltage source which is connected by said first and secondcircuit means to said metering means and said first and second electrodemeans for establishing said primary, auxiliary, and tertiary electricfield.
 14. The electrostatic sprayer of claim 1 wherein said article isa dielectric, and further including a means positioned at an input sideof said sprayer for depositing a predetermined charge on said articlewhereby said auxiliary electric field is established between saidmetering means and said article, and a means positioned at an outputside of said sprayer for discharging said article.
 15. The electrostaticsprayer of claim 1 wherein one of said metering means and said conductormeans is insulated.
 16. A method for electrostatically spraying a powdercoating on an article comprising the steps of: continuously supplyingparticles of powder, exposing said particles in a predetermined area ata distance from said article to an a.c. primary electric field so thatsuccessive groups of particles are oppositely charged relative to apredetermined reference potential for said primary field, and creatingan auxiliary electric field to control movement of said particles for atleast a portion of said distance.
 17. The method of claim 15 whereinsaid auxiliary electric field extends for at least the entirety of saiddistance, wherebY it controls movement of said particles from the areaof said primary field to said article.
 18. The method of claim 16wherein said auxiliary electric field asserts a force on said particlesin a direction that is angularly displaced from the direction of anygravitational forces acting thereon.
 19. The method of claim 15 furtherincluding the step of moving successive articles to be coated past apredetermined spraying station at a predetermined rate; and wherein saidauxiliary electric field is an a.c. field, and said primary andauxiliary fields have relative frequencies and phases that are selectedto cause said powder particles to move toward and away from said stationin timed synchronism with the movement of successive articles thereby.